The present invention relates to level detection and sensing through the technique of monitoring variations in the mutual inductance of coupled inductors.
Heretofore, level detection devices having level sensors formed by inductively coupled coils have been utilized in atomic energy work to monitor the level of liquid metal coolant employed in nuclear reactors. One liquid metal presently employed as a coolant in nuclear reactors is the electrically conductive material liquid sodium, and such inductive level sensors have been found to be particularly useful for measuring the surface level of liquid sodium in nuclear reactors.
The operation of such inductive level sensors is based on the fact that in a detecting element formed by inductively coupled primary and secondary coils the mutual inductance between the two coils is dependent upon the electrical material surrounding the coils. In particular, if the primary and secondary coils are surrounded by a conductive material, such as liquid sodium, the mutual inductance between the two coils will be substantially different than if they were surrounded by air.
Accordingly, the level of a pool of liquid sodium can be monitored by placing such an inductive level sensor so that the extent its coils are surrounded by liquid sodium varies proportionally as a function of pool level and driving its primary winding to induce an output across its secondary coil. In accordance with Faraday's law of induction, the induced output of the secondary coil is directly proportional to the mutual inductance between the primary and secondary coils. Since the mutual inductance between the two coils is proportional to the fractional extent the coils are surrounded by liquid sodium, which in turn is directly proportional to the level of the pool of liquid sodium, it is thus apparent that the induced output of the secondary coil varies as a function of pool level and can be utilized to provide a measurement of the level of the pool of liquid sodium. In absence of variations in the conditions under which the inductive level sensor is operating, the induced output of the secondary coil provides a measurement of pool level which is substantially linearly proportional.
In the operation of a nuclear reactor, however, the conditions under which the inductive level sensor operates do not remain constant. In particular, the temperature range in which the inductive level sensor operates varies substantially due to the fact that the temperature of the liquid sodium coolant typically varies during reactor operation from a low of 300.degree. F. to a high of 1200.degree. F. Such operating temperature variations, in addition to causing changes in the electrical resistance of the inductive coils making up the level sensor, have also been found to cause major variations in the coils mutual inductance parameter, i.e., the mutual inductance between the coils is highly temperature sensitive and dependent.
Heretofore, attempts have been made to compensate for or eliminate the effects of temperature variations on the operation of such inductive level sensors. For example, see the following U.S. patents: Marinaccio U.S. Pat. No. 3,896,671; Kokayashi et al U.S. Pat. No. 3,834,234; and, Cambillard et al U.S. Pat. No. 3,402,607. Such attempts have not proven to be entirely satisfactory, and accordingly, the problem of operating temperature variations effecting level measurements made by such inductive level sensors has continued to be a problem of major concern in the atomic energy industry.